1. Technical Field
The present invention relates to an electro-optical device that uses an electro-optical material, such as liquid crystal, to a method of driving an electro-optical device, and to an electronic apparatus having an electro-optical device.
2. Related Art
In general, electro-optical devices, such as liquid crystal display devices for displaying images or the like, have been known. This electro-optical device includes, for example, a liquid crystal panel, and a driving circuit that drives the liquid crystal panel. For example, the electro-optical device has the following structure.
The electro-optical device includes a liquid crystal panel, a liquid crystal driving circuit that drives the liquid crystal panel, and a control circuit that controls the liquid crystal driving circuit. The liquid crystal driving circuit includes a scanning line driving circuit, a data line driving circuit, and a liquid crystal driving power supply circuit.
The liquid crystal panel includes an element substrate, a counter substrate, and liquid crystal serving as an electro-optical material. In the element substrate, thin film transistors (hereinafter, simply referred to as TFTs), which serve as switching elements (which will be described in detail below), are disposed in a matrix. The counter substrate is disposed so as to be opposite to the element substrate, and the liquid crystal is interposed between the element substrate and the counter substrate.
The element substrate includes a plurality of scanning lines that are provided at predetermined gaps, a plurality of data lines that are provided at predetermined gaps in a direction substantially orthogonal to the plurality of scanning lines, and capacitor lines that are disposed substantially parallel to the plurality of scanning lines and provided alternately with the plurality of scanning lines.
A plurality of pixels are provided at intersections between the plurality of data lines and the plurality of scanning lines. Each of the pixels includes a pixel electrode and a storage capacitor that has one end connected to the pixel electrode and the other end connected to the capacitor line, in addition to the above-mentioned TFT.
The TFT has a gate that is connected to the scanning line, a source that is connected to the data line, and a drain that is connected to the pixel electrode and the storage capacitor.
In the counter substrate, a plurality of common lines are provided so as to be substantially parallel to the plurality of scanning lines. In the counter substrate, a common electrode is formed so as to be opposite to the pixel electrodes. The common electrode is connected to the common line.
The electrode-optical device having the above-mentioned structure operates as follows. That is, a control signal is line-sequentially supplied, so that all of the pixels corresponding to a predetermined scanning line are selected. In addition, an image signal is supplied to the data line in synchronization with the selection of the pixel. Thereby, the image signal is supplied to all of the pixels selected by the control signal, so that the image data is written in the pixel electrode.
In the electro-optical device, in a state in which a voltage of the common electrode is used as a reference voltage, positive polarity writing, in which an image signal is supplied to the data line with a voltage greater than the voltage of the common electrode, and negative polarity writing, in which an image signal is supplied to the data line with a voltage smaller than the voltage of the common electrode, are alternately performed.
If the image signal is written in the pixel electrode of the pixel, a driving voltage is applied to the liquid crystal due to the potential difference between the pixel electrode and the common electrode. Accordingly, a voltage level of the image signal is changed, so that the alignment or order of the liquid crystal molecules is changed, which results in gray-scale display through optical modulation of each pixel.
In addition, the driving voltage that is applied to the liquid crystal is held by the storage capacitor for a longer time, namely, for a period as much as three orders of magnitude longer than the time for which the image signal is written.
In this case, in the electro-optical device having the above-mentioned structure, a parasitic capacitance is generated between the gate and the drain of the TFT. Also, a parasitic capacitance is generated between the source and the drain of the TFT. If the gate voltage of the TFT enters an off state, an electric charge accumulated in the storage capacitor and an electric charge accumulated in a pixel capacitor consisting of the pixel electrode and the common electrode are redistributed while including parasitic capacitances, which results in generating so-called pushdown that the voltage of the pixel electrode is reduced and the voltage applied to the liquid crystal is also reduced.
In both the positive polarity writing and the negative polarity writing, the pushdown is always generated. Therefore, a center voltage Vc, which is a center value between a positive polarity image signal and a negative polarity image signal, is set higher than the voltage VCOM of the common electrode by a dropped voltage due to the pushdown.
In the meantime, the electro-optical device having the above-mentioned structure is used in, for example, a portable apparatus. In recent years, the portable apparatus has been required to further reduce a consumed power. Accordingly, display is performed on an entire screen of a display screen (hereinafter, this case is referred to as an entire screen display mode), and display is performed on only a part of a display screen (hereinafter, this case is referred to as a partial display mode), which results in a decrease of a consumed power (for example, see JP-A-2001-356746).
In the electro-optical device disclosed in JP-A-2001-356746, in the partial display mode, a display screen is divided into a display region and a non-display region. On the display region, a remaining battery capacity or time is displayed, and nothing is displayed on the non-display region. That is, in a normally white mode, the non-display region is displayed with a white color, and in a normally black mode, the non-display region is displayed with a black color.
In the above-mentioned partial display mode, similar to the above-mentioned entire screen display mode, if a center voltage Vc of the image signal is set higher than a voltage VCOM of the common electrode by a dropped voltage due to the pushdown, even though nothing is displayed on the non-display region, the potential difference is always generated between the pixel electrode and the common electrode due to the pushdown, which results in an increase of a consumed power.
Accordingly, in the partial display mode, in order to reduce the consumed power in the non-display region, the center voltage Vc of the image signal is set so as to be the same as the voltage VCOM of the common electrode. Thereby, the consumed power is reduced in the non-display region.
However, in the partial display mode, if the center voltage Vc of the image signal is the same as the voltage VCOM of the common electrode, even through the consumed power in the non-display region can be reduced, in the display region, the center voltage Vc of the image signal becomes smaller than the voltage VCOM of the common electrode by an offset potential due to the pushdown. Therefore, burning or flickering may occur on the display region.